The invention relates to antennas. More particularly, the invention pertains to compact, multi-band antennas.
As more and more wireless or radio frequency (RF) services become available to the general public, the need for compact antennas increases. The size and configuration of antennas typically is not of great concern for stationary applications, but becomes a significant issue in connection with mobile applications. For instance, it is not uncommon now for an automobile to have multiple built-in wireless/RF devices, including, but not limited to, a cellular telephone, a global positioning satellite (GPS) system for navigational purposes, and a digital satellite radio/audio system. Most modern cellular telephones are themselves tri-mode telephones capable of transmitting and receiving in three distinct bands, namely, an analog band which operates in a band of 824-896 MHz, a digital band in accordance with the American Mobile Phone System (AMPS) protocol which operates in a band of 806-896 MHz, and a second digital band in accordance with the Personal Communication Systems (PCS) protocol which operates in a band of 1850-1990 MHz.
In monopole antenna design, an antenna mast typically extends perpendicularly from a ground plane (or ground plate). In accordance with international standards, the antenna should present a purely resistive 50 ohm impedance at its input terminal in the frequency band in which it is intended to receive and/or transmit. This can be accomplished by providing an antenna mast of a length that has good resonance at the frequency of the signals it is to receive and/or transmit. In simple monopole antenna designs, a mast that is approximately equal in length to one quarter wavelength of the signals it is to transmit and/or receive has good resonance and provides a very good input match to 50 ohms.
However, it is often impractical or even impossible to provide an antenna mast having a length equal to one quarter of a wavelength. At a minimum, it is almost always desirable to reduce the size of all electronics related components, including antennas and particularly antenna masts, especially in mobile (e.g., cars) or hand-held (e.g., cellular telephone, wireless personal digital assistant) applications.
It is well known to xe2x80x9ctop loadxe2x80x9d monopole antennas in order to reduce the required length of the mast. Particularly, if a second conductive plate is placed at the distal end of the antenna mast generally perpendicular to the ground plane, resonance can be achieved with a much shorter antenna mast. Particularly, top loading a monopole antenna introduces a capacitance between the top plate and the ground plane that, in accordance with well known antenna theory, substantially reduces the required length of the antenna mast (the spacing between the top plate and the ground plane) needed to achieve resonance for a particular frequency of electromagnetic wave. Despite the capacitance between the ground plate and the top plate, the device still reasonably emulates a 50 ohm impedance.
Another common type of antenna is known as a microstrip antenna. A microstrip antenna commonly comprises a sheet of material with good microwave properties and appropriate thickness and having copper cladding on both sides. The sheet may take any number of shapes but is usually a square having a size that is determined as a function of the wavelength of interest. A portion of the copper cladding on one side is usually etched away to a predetermined size. Microstrip antennas radiate from their edges and are very compact. However, they typically have very narrow effective bandwidths and thus typically are suitable only for use with receivers, transmitters and/or transceivers that operate over a very narrow bandwidth. GPS would be a good example of a protocol in which microstrip antennas can be used effectively since the bandwidth for GPS transmissions is very narrow.
It also is common for microstrip antennas to be sold as an integral unit with a printed circuit board having active circuitry thereon. Particularly, the microstrip antenna may be attached on the top side of a printed circuit board, for instance, by double sided adhesive tape, with active circuitry disposed on the bottom side of the printed circuit board. The bottom of the printed circuit board is then covered with an enclosure, commonly called a xe2x80x9ccanxe2x80x9d, in order to protect the circuitry.
It is an object of the present invention to provide an improved multi-band antenna assembly.
It is another object of the present invention to provide a multi-band antenna assembly that is compact.
It is a further object of the present invention to provide an efficient multi-band antenna with high gain.
The invention is a multi-band antenna in which two, three or more antennas are contained within a single housing/radome. In accordance with a first aspect of the invention, two top-loaded monopole antennas are nested together with one of the antennas being positioned between the ground plate and top plate of the other antenna. Inductive shunts for counteracting the capacitance in the two top-loaded monopole antennas can be provided by hollow conductive tubes in order to help the antenna more closely emulate a purely resistive 50 ohm impedance. In accordance with another aspect of the invention, a third, microstrip antenna may be positioned on top of the top conductive plate of the outer top-loaded monopole antenna. The cable for the microstrip antenna is routed through the ground plate and top plate of at least one of the top-loaded antennas and through the inside of one of the hollow inductive shunts.